The present invention relates to an image forming apparatus of an electrophotographic system, such as a copying machine or laser printer, and more particularly, to an image forming apparatus having a belt-shaped photosensitive member.
An electrophotographic apparatus, such as an electrophotographic copying machine or electrophotographic printer, generally comprises; a photosensitive member, such as a photosensitive drum or a photosensitive belt; a charging unit for uniformly charging the surface of the photosensitive member; an exposing unit for exposing the surface of the photosensetive member to light in accordance with image information, thereby forming an electrostatic latent image on the charged surface of the photosensitive member; a transfer unit for transferring a toner image to a recording sheet; and a thermal fixing unit fixing the toner image onto the recording sheet, wherein the recording sheet on which the toner image has been transferred passes through the thermal fixing unit.
In an image forming apparatus of the electrophotographic type using a photosensitive belt, the photosensitive belt has generally a joint so as to be endless. When the joint passes the cleaning unit, the cleaning unit is vibrated due to a difference in level at the joint. Because of the vibration, a photosensitive belt unit is also vibrated. When a latent image is formed by the exposing unit, the latent image is consequently vibrated so that a density variation in a lateral line shape which is perpendicular to the paper conveyance direction occurs on the image. Since the density variation remarkably deteriorates the quality of the image, the vibration of the cleaning unit has to be prevented in order to prevent such a density variation. Therefore, various processing methods for reducing the difference in level at the joint of the photosensitive belt have been conventionally proposed.
For example, Japanese Patent Application Laid-Open No. 1-288860 discloses a method in which the ends of a belt are connected by ultrasonic fusion and an extruded length from the joint portion is set to 1.2 mm or shorter. Japanese Patent Application Laid-Open No. 61-185753 discloses a method of forming a protective film including a solid lubricant in the joint portion.
According to the conventional methods, as mentioned above, the latent image is vibrated by the vibration which occurs when the photosensitive belt passes the cleaning unit due to the joint in the photosensitive belt. The joint is the cause of occurrence of a density variation in the image, so that the joint is processed to reduce the difference in level in order to prevent the vibration. Although the density variation in the image is suppressed by reducing the difference in level by processing the joint, all of the variation is not eliminated. The process cannot be perfectly executed by variation in processing works.